This invention relates to communications systems, and in particular to troposcatter communications systems.
Troposcatter radio links have been in use for more than 60 years. The early systems used FM/FDM modulation which were limited to a couple of dozen voice channels. These links were used by commercial carriers across the southern USA. The DEW line in Alaska, Northern Canada and Greenland used Tropo radios also. Many militaries have Troposcatter radio systems. Troposcatter links are used to communicate to offshore locations such as oil platforms or offshore islands. These systems can transmit data over the horizon usually at distances from 25 to 200 miles; a few links operate up to 400 miles. The advantage of Troposcatter systems is that they can be quickly set up and they provide reliable broadband data links over the horizon. The main competition for Troposcatter links are satellite links. These have displaced many Troposcatter links. However satellite links are very expensive to put up and are expensive to rent. It also takes a while to put a satellite in orbit. In many situations Troposcatter links are more economical. The medium for troposcatter communications is the scattering of radio waves from the upper part of the troposphere this is a layer from ground level to about twelve miles above the earth. Scattering is only detectable when the radio beams are both narrow and both close to horizontal. FIG. 1 shows the path. The antenna beam width is much narrower than shown and the elevation angles are exaggerated for clarity. Transmit and receive antennas are each pointed at a volume of the troposphere above the midpoint of the path. The path loss increases quickly as the beams are pointed away from the optimum position so the two beams must be pointed accurately. The scattering results from thermal inhomogeneity in the air at that elevation of the common volume. The path loss is very high so the transmit power must be high and the antennas must be large. In most cases the signal is sent four times in parallel from two transmitters at different frequencies and is received at both frequencies by each of two receive antennas so the receiver has four copies of the transmit signal. The distortion on each path is different and independent of the others. If these four copies (diversities) are properly combined the signal can be reliably detected. This is called four diversity reception. Some troposcatter systems receive just two copies of the signal. These Dual diversity systems are lighter and more portable but have less range and less reliability. There are a few other ways to set up a troposcatter link. The set up described above is a 2 Frequency/2Space system. In some cases angle diversity can be used. Angle diversity uses two feeds on each antenna so that two beams can be formed at slightly different angles from one antenna usually one beam above the other. The common volume of the upper beams is different from the common volume of the lower beam so the scattering is different in the two beams. This means that they are statistically independent and can be combined in the same way as described above. Angle diversity is generally less robust than two frequency/two space systems because neither beam is pointed at the optimal spot. Usually the lower beam is pointed somewhat into the ground and the upper beam is pointed above the optimum angle.
A troposcatter link can also be set up using a single transmit antenna and power amplifier at each end and four receive antennas, one of which can be transmit antenna. This would be a 1 Frequency/4Space system. This invention works with any known diversity scheme.
In the 1970s and 1980s digital troposcatter modems were developed these were a major advance, data rates up to 12 Mb/S were possible. These modems operated in the time domain. The accuracy of the equalization was limited. Today the similar techniques have resulted in modems that operate at rates of up to 20 Mb/S.
Troposcatter radio modems are used for communications over distances longer than the radio horizon. This is possible by using the scattering of the troposphere. A small amount of the signal power is scattered toward the receiver when the radio wave passes through the troposphere at roughly 10 miles above the earth's surface. This requires large antennas and high power amplifiers at each end of the link. The scattering process causes the signal to be dispersed (spread out) in time so that multiple copies of the signal arrive at the receiver. These ‘multipath’ signals interfere with one another and cause the signal to fade continuously which can cause errors. To mitigate this effect the links are usually built with two antennas and two power amplifiers at each end operating at different frequencies, resulting in a four diversity system. The receiver has four independently fading copies of the signal. By properly combining these, the performance can be greatly improved. However the multipath effect limits the maximum data rate to about 20 Mb/S using existing troposcatter modems. The reason for this is that the power must be increased as the data rate is increased and the signal processing becomes more and more difficult as the data rate increases. This invention addresses the signal processing problem by using a new novel frequency domain technique to make the signal processing difficulty much less dependent on the data rate and the amount of dispersion. This invention addresses the power limitation by using a new technique to improve diversity combining, to adapt the data rates to the channel conditions and to use modern coders in a way that balances the error rate requirements with the power available.
In combination the techniques disclosed herein, will allow several advances in troposcatter communications, the possible data rates data rates will be much higher than earlier systems and the error rates can be easily controlled to provide the highest data rate for a given max error rate.
It will also allow greatly improved performance in dual diversity or single diversity systems making lighter and less expensive systems possible.